Since close to the beginning of the century, insulation measurements have been made using techniques which have been highly developed and instruments built specifically for that purpose such as the Megger.RTM. sold by James G. Biddle Company. Such instrumentation requires a high degree of knowledge and skill on the part of the operator in order to obtain readings which are reliable and, by following a painstaking procedure, can achieve a high degree of accuracy in measuring insulation resistance. Persons with the skill and patience to practice this method are becoming relatively unavailable.
In 1958, E. B. Curdts made a technical analysis of insulation testing entitled "Insulation Testing By D-C Methods" which he revised and reprinted in 1964 in Biddle Technical Publication 22T1. In that publication, Curdts showed that when d.c. voltage is applied, the current existing in the insulation of a capacitive specimen is always made up of three components, to wit: geometric capacitance current, i.sub.g ; absorption current, i.sub.a ; conduction or leakage current, i.sub.z. These currents will be explained more fully hereafter, but it will be understood that the current which was measured by all conventional insulation testers was i.sub.TOT, EQU where i.sub.TOT =i.sub.g +i.sub.a +i.sub.c.
Because the i.sub.TOT is measured by all conventional insulation testers, various elaborate techniques have been evolved to estimate the value of i.sub.c in the presence of i.sub.g and i.sub.a. Typical examples are time-resistance tests, step-voltage tests, polarization index and dielectric absorption ratio tests. These tests have been developed to a considerable degree of sophistication and achieve excellent results when applied by skilled technicians. However, they suffer from the disadvantages of taking comparatively long periods of time and requiring great care and skill in the measurements and their interpretation.
In 22T1, previously referenced E. B. Curdts has described a method of calculating i.sub.c. He shows that since the absorption current is a power function of time, constant n (with a value between 0 and 1.0) is the slope of the straight-line current-time curve plotted on a log-log basis.
The leakage current i.sub.c will represent a deviation from this curve, and may be calculated from ##EQU2## where i.sub.1, i.sub.3.16 and i.sub.10 are three values of i.sub.TOT measured at different times, based on a constant unit of time multiplied by their subscripts, i.e., 1, 3.16 and 10 minutes.
This relationship is true provided that:
a. i.sub.g has fallen to a negligible value compared to i.sub.a and i.sub.c ; which therefore also assumes that
b. i.sub.g is negligible at that voltage level when compared to i.sub.a and i.sub.c.